I agree with @robuspierre. We're talking about three different things here:
Chives are a herb - we only eat the part that grows above ground ie we eat the shoots not the bulbs. The part we eat looks like dark green grass with thin tubular shoots. You'd typically cut chives into small pieces and sprinkle on top of a dish just before serving to give a fresh, onion taste. Sour cream and chives is a popular topping for baked potatoes.
Spring onions and scallions are alternative names for the same thing. We tend to say spring onions in Britain and scallions in the US. They have a small white bulb and long shoots which are white at the base and dark green at the tips. You can eat them raw in salads, or cooked in Asian-style stir-fries.
Shallots are small onions, maybe 5 cm long by 3 cm wide, often reddish in colour. As with ordinary onions, we only eat the bulbs of shallots.